<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<script type="text/javascript">
    window.onload = function () {
        var osel = document.getElementById("selID"); //得到select的ID
        var opts = osel.getElementsByTagName("option");//得到数组option
        var obt = document.getElementById("bt");
        obt.onclick = function () {
            opts[3].selected = true;//设置option第4个元素，即value="3"为默认选中
        }
    } 
</script>

<body>
    <select name="select" id="selID">
        <option value="0">下拉菜单一</option>
        <option value="1">下拉菜单二</option>
        <option value="2" selected>下拉菜单三</option>
        <option value="3">下拉菜单四</option>
        <option value="4">下拉菜单五</option>
    </select>
    <input type="button" id="bt" value="查看效果" />
</body>

</html>